Code Division Multiple Access (CDMA) is a spread-spectrum communication technology that has become increasingly popular in mobile wireless communications systems, e.g., digital cellular radio systems. In a CDMA system, the time and frequency domains are simultaneously shared by all users as a base station simultaneously transmits distinct information signals to multiple subscriber mobile stations over a single frequency band. CDMA systems have a number of advantages over other multiple access systems, e.g., Frequency Division Multiple Access and Time Division Multiple Access, such as increased spectral efficiency and, as discussed below, the ability to mitigate the effects of signal fading using path diversity techniques.
Prior to transmission, a CDMA base station multiplies the individual information signal intended for each of the mobile stations by a unique signature sequence, referred to as a pseudorandom noise (PN) sequence. This PN sequence can be formed by multiplying a long pseudorandom noise sequence with a time offset which is used to differentiate the various base stations in the network, together with a short code unique to each mobile station, for example, the Walsh codes. The multiplication of the information signal by the signature sequence spreads the spectrum of the signal by increasing the rate of transmission from the bit rate to the chip rate. The spread spectrum signals for all subscriber mobile stations are then transmitted simultaneously by the base station. Upon receipt, each mobile station de-spreads the received spread spectrum signal by multiplying the received signal by the mobile station's assigned unique signature sequence. The result is then integrated to isolate the information signal intended for the particular mobile station from the other signals intended for other mobile stations. The signals intended for the other mobile stations appear as noise. The structure and operation of CDMA systems are well known. See, e.g., Andrew J. Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison-Wesley Publishing, 1995; Marvin K. Simon, Jim K. Omura, Robert A. Scholtz, and Barry K. Levitt, Spread Spectrum Communications Handbook, McGraw-Hill, Inc., 1994.
One advantage of CDMA systems over other multiple-access telecommunications systems is the ability of CDMA systems to exploit path diversity of the incoming radio-frequency (RF) signal. The CDMA signal, including a pilot signal and traffic signals between a base station and mobile stations, is communicated from a transmitter to a receiver via a channel including several independent paths, referred to as multiple signals or “multipaths”. Each multipath represents a distinct route that the information signal takes between the transmitter and receiver. The transmitted signal thus appears at the receiver as a plurality of multipath signals or multipaths. Each multipath may arrive at the receiver with an arbitrary timing delay, and each multipath may have a different signal strength at any time due to signal fading.
CDMA systems employ “RAKE” receivers in mobile units and base stations to exploit this path diversity. RAKE receivers estimate the timing delay introduced by each of one or more multipaths in comparison with some reference, e.g., line-of-sight delay, and then use the estimated timing delay to receive the multipaths which have the highest signal strength. A typical RAKE receiver includes a plurality of RAKE branches or “fingers”, typically two to six fingers. Each finger is an independent receiver unit, often referred to as a correlator, which assembles and demodulates one received multipath which is assigned to the finger. A RAKE receiver also includes a separate “searcher” which searches out different signal components of an information signal that was transmitted using the assigned signature sequence of the receiver, and detects the phases of the different signal components. The timing of each finger is controlled such that it is correlated with a particular multipath which arrived at the receiver with a slightly different delay, as was found by the searcher in its receipt of the information signal. Thus, each finger is “assigned” to a particular multipath by controlling its timing to coincide with arrival of the multipath. The demodulated output from each finger, representing one multipath, is then combined into a high-quality output signal which combines the energy received from each multipath that was demodulated. The implementation of RAKE receivers is generally known for both forward and reverse CDMA channels. See, e.g., R. Price and P. E. Green, Jr., A Communication Technique for Multipath Channels, 46 Proc. Inst. Rad. Eng. 555-70 (March 1958); G. Cooper and C. McGillem, Modern Communications and Spread Spectrum, Chapter 12, McGraw-Hill, NY, 1986.
Finger lock algorithms are used to determine if signals of correlators of fingers in the RAKE should be used in a RAKE receiver combiner. Finger lock algorithms are based on various estimates of signal qualities. Typical finger lock algorithms are based simply on the pilot signal strength, such as an estimate of the ratio of pilot energy determined for pilot signal chips to interference received at the mobile station (Ec/Io) and measured by a finger, which indirectly is an estimate of the pilot energy determined for pilot signal chips to interference transmitted at the base station (Ec/Ior). For example, the Ec/Io of each finger in a RAKE receiver is estimated and used to determine if the finger should be used in combining. The determination of whether to use the finger or not is based upon a signal quality threshold. If the estimate Ec/Io of a finger is above the threshold, the finger is locked, meaning the signal on that finger path is used in the combiner. If the estimate Ec/Io of a finger is below the threshold, the finger is unlocked and the combiner will not use the data from the finger.
The threshold is determined to prevent adding noise to the combined signal. Thus, the threshold is typically established based upon a desired signal strength above a noise level. The result being that signal data is combined from any finger which can help to increase the combined SNR. If no signal exists on a path, having the finger locked would reduce the output SNR of the combiner. However, if a signal exists on an unlocked finger, the information would be lost to the combiner, reducing the output SNR of the combiner. One or more threshold values may be used for a logical decision to lock or unlock a finger. If a single threshold value is used, a finger is locked if its signal strength is estimated to be above the threshold and unlocked if below the threshold. To prevent a finger from fluctuating between lock and unlock status, two thresholds may be used, where a lock threshold is set greater than an unlock threshold and the finger remains in the current lock or unlock status between the two thresholds. For instance, if the finger is in an unlock position, the finger is not locked until the signal strength estimate reaches the higher lock threshold, and once the finger is locked, the finger is not unlocked until the signal strength estimate drops to the lower unlock threshold.
However, the signal strengths of pilot and traffic signals between a base station and mobile stations may vary and the ratio of the pilot signal strength to traffic signal strengths may vary. For example, the signal strengths of a pilot channel may remain constant, but the signal strengths of traffic channels may change based upon forward link power control bits sent by a mobile station to maintain a particular level of service at the mobile station. Thus, the forward traffic channel gain (FTCG) at the base station may constantly change. Similar signal strength of traffic channels vary when Fast Forward Power Control (FFPC) is enabled or with certain IS-2000 Forward Radio Configurations. For example, the pilot signal may be weak, but the traffic channel may be very strong and could contribute to the output SNR of a RAKE receiver combiner. Use of power measurement report messages (PMRM) in IS-95 could also result in strong traffic channel transmission signals in weak signal conditions. Thus, even in situations when a pilot signal strength may be very weak, significant and sufficient signals may be available on some multipaths due to forward power control which could improve demodulating forward link data. Further, typical RAKE receiver finger lock algorithms which are based on pilot strength estimates and thresholds set according to pilot strength estimates may result in fingers being unlocked when information on at least some multipaths could be used by a RAKE receiver combiner to increase output SNR.
Accordingly, there is a need in the art for a system and method for improved finger lock status determination for RAKE receiver combiners, particularly for use with fast forward power control systems.